The present invention relates to selective one or two-sided coating of a continuous strip tape or the like, and more particularly the invention relates to zinc plating or coating of one or both sides of a steel strip.
Equipment of the type to which the invention pertains will include a coating or layering station proper preceded by a protective gas zone, and including a deflection pulley roll or the like rotatably mounted on a horizontal axis in the range of the coating material; further are including devices for the control of the thickness of the coating.
The demand for one or single side coated, particularly zinc-coated steel strip, has increased considerably in the recent past. This kind of material is used to a considerable extent for sheet stock used for making automobile parts, whereby particularly lacquer, varnish or other nonmetallic, usually synthetic coating or the like will be deposited on that side of the steel strip which is not zinc coated. The zinc coating is situated on the inside of the cabin or vehicle compartment made from such steel sheet stock and improves considerably the corrosion resistance of the sheet which in turn enhances considerably the life of the vehicle.
Modern zinc coating plants are usually designed for such a high throughput that the one sided coating will not be provide for sufficiency occupancy of the plant. Steel strip with a zinc coating on both sides is still in high demand. Therefore, the problem has arisen to provide equipment and zinc coating facilities which can be selectively operated for one or two sided coating.
In accordance with a known equipment and plant of the latter type, it has been suggested to run the stock to be coated along a lower peripheral portion of a rotatably mounted deflection roll which dips into the liquid metal. The deflection roll is wider than the strip material. The transition from a one sided to two sided coating and vice versa, is carried out in that the position of the line of contact between sheet stock and deflection roll is varied in relation to the surface level of the coating bath. If the strip or sheet engages the deflection roll already ahead of the point in which the roll dips into the liquid metal, then only the outer surface of the sheet stock will be coated; the side surface of the strip engaging the deflection roll is protected against coating thereby. On the other hand, if one lowers the deflection roll deeper in the bath and/or the sheet stock approaches the deflection roll at a shallower angle and is also taken up from the roll at a correspondingly shallower angle, layering and coating material will be able to penetrate into the more or less triangular space between the roll and that side of the strip which will be, but is not yet covered by, the roll following engagement. This way one does obtain a two-sided coating.
However, it was found that this particular kind of equipment is disadvantaged in those cases in which one is not in fact able to keep this one side of the strip or sheet clean from coating material. Through capillary action or otherwise, coating material may creep into the gap between the sheet and the deflection roll, particularly near and along the edges of that sheet material. However, the one sided coating does not merely require that the one side be basically free from coating material, it must be completely free from such a material, particularly because of the subsequently employed lacquering or varnishing of the uncoated side. See here for example German printed Patent Application 3,045,403.
Generally speaking equipment for the double or two-sided coating of a continuous running strip is usually comprised of a container in which is contained the liquidous coating material and one or two rolls are provided for rotation about horizontal axes, these rolls are journalled accordingly in the container, and the material to be coated is drawn under these rolls. Equipment of this type generally is disclosed, for example in German printed Patent Application 1,521,159a (corresponding U.S. patent application Ser. No. 445,201 of Oct. 2, 1965).
Additional equipment is known for the one-sided coating of strips which does not exhibit the drawbacks of that particular equipment mentioned above which is provided for the selective one and two sided coating. Thus, coating on one side only is carried out by equipment which makes sure that indeed only one side receives coating material, and this critical aspect is attained by running the strip above the coating medium around a horizontally journalled deflection roll, and by providing a scooping roll below that roll which dips into the coating material and runs otherwise axis parallel to the deflection roll. Equipment of this type is disclosed, for example, in German printed Patent Application 3,009,590.